Field of the Invention
The present invention relates to an ionomer resin composition, an optical film, a polarizing plate, and a liquid crystal display apparatus.
Background Art
Ionomer resins refer to resins having a special structure in which acidic group-containing polymer molecules are bonded to each other by a metallic ion such as sodium or zinc, such as ethylene-methacrylic acid copolymers and ethylene-acrylic acid copolymers. In addition, since ionomer resins are excellent in heat sealability, transparency, toughness, abrasion resistance, low-temperature resistance, and the like, they are used for a wide variety of applications, such as golf ball covers, ski shoes, packaging materials of food and the like, molded products, films, molding for cars, and top lifts for ladies' shoes.
Film-shaped optical members (such as, for example, a film used in a liquid crystal display apparatus and a substrate of a prism sheet) used in various kinds of optics-related instruments are generally referred to as “optical films”. Optical films are required to have not only transparency but also optical characteristics strictly adjusted so as to be adaptable to optical design of image display apparatuses). One of important optical characteristics of such optical films is birefringence. In other words, it is sometimes not preferable that optical films have high birefringence. Particularly, in IPS-mode liquid crystal display apparatuses, the use of a film having high birefringence can cause negative influence on image quality. Accordingly, an optical film having low birefringence is desirably used as a protecting film or the like for a polarizing plate used in liquid crystal apparatuses.
In addition, since large screen image display apparatuses that have been recently increasingly developed use a light source with high brightness, an amount of heat generation thereof is on the increase. In order to dispose an optical film near such a light generation unit, the optical film needs to have high heat resistance. A film having high heat resistance can be achieved by using a resin having high glass transition temperature (Tg). However, such a film is fragile and easily broken. In other words, optical films are required to have high heat resistance and flexibility.
Japanese Unexamined Patent Application Publication No. 2011-242754 discloses an optical film including a (meth)acrylic polymer having an N-substituted maleimide unit and a (meth)acrylate ester unit as structural units.
Japanese Unexamined Patent Application Publication No. H6-25359 discloses a low birefringent material (such as a film, a sheet material, a lens, or an optical disc) including from 50 to 92 parts by mass of a methyl methacrylate unit, from 5 to 20 parts by mass of a specific benzyl methacrylate, from 3 to 20 parts by mass of a specific maleimide-based monomer, and from 0 to 10 parts by mass of another polymerizable vinyl monomer unit.
International Publication WO 2009/123138 discloses an ionomer resin composition that is obtained by reacting 10% by mole or more of a cyclic olefin-derived structural unit and a functional group-containing olefin copolymer having a group derived from an acid and/or a derivative thereof as a functional group with a metal compound.